An MRI apparatus measures NMR signals generated by atomic nucleus spins comprising an object, particularly, human tissue to two-dimensionally or three-dimensionally image forms and functions of the head, abdomen, limbs, and the like. In imaging, the object is placed in a static magnetic field (a polarizing magnetic field Bo), a high-frequency magnetic field pulse is applied together with a slice-selective gradient magnetic field pulse in order to selectively excite a certain region, and then an excitation range is encoded to provide positional information by applying a phase encoding gradient magnetic field pulse and a frequency encoding gradient magnetic field pulse. The measured NMR signals are two-dimensionally or three-dimensionally Fourier-transformed in order to reconstruct an image.
Generally, in the MRI apparatus, by amplifying the NMR signals immediately after they are received by a reception coil, a ratio of noise to be mixed in the reception system later can be reduced relatively. At this time, gain that amplifies the NMR signals is referred to as reception gain. Since the ratio of noise to be mixed is more reduced as the reception gain is greater, an image with a high SNR can be obtained in proportion to the increase of the reception gain. Also, since a quantization error is more reduced when the NMR signals are converted into digital signals as the reception gain is greater, a highly accurate image can be obtained.
However, actually, because an A/D converter has a dynamic range limitation, a magnitude of the reception gain to be applied to the NMR signals is limited. Especially in case of a spin echo sequence, there is a timing at which phases of the NMR signals in each position are aligned, a dynamic range of an NMR signal to be received at the timing is large. Therefore, in order to correspond to the NMR signals at this timing, the reception gain of the A/D converter cannot be increased.
In order to solve this problem, there is one method of obtaining the NMR signals with a wide dynamic range by changing the reception gain to measure the NMR signals a plurality of times and synthesizing the respective measurement signals. Also, there is a method of using companding (for example, Non-patent Literature 1). In this method, analog NMR signals before AD conversion are transmitted through a non-linear amplifier (for example, a logarithmic amplifier) to perform non-linear compression processing. Then, expansion processing is performed for NMR signals after AD conversion according to the characteristics of the amplifier.